The use of measuring apparatus incorporating microprocessors is gaining increased acceptance, with their fields of application enjoying enormous expansion. This extensive recourse to microprocessors in measuring apparatus is to be ascribed to their low cost and highly versatile features. There are, however, situations in which the skill of the operator taking the readings--especially facing "vague" decisions that the operator is expected to make--is still regarded as an irreplaceable contribution.
Medical applications show several examples where such vague decisions are to be made which restrict the usability of fully automated measuring apparatus. Specifically, the measuring of arterial blood pressure and heart beat frequency will be considered in the following description by way of example. As is well known, the measuring of arterial blood pressure is aimed at checking a patient's maximum or systolic pressure and minimum or diastolic pressure. Also known is that the heart beat frequency is usually expressed as beats per minute in the medical field.
For measuring arterial blood pressure, there are basically two measuring methods available: a direct or invasive type of measurement, and an indirect or non-invasive type of measurement. The invasive direct method involves the insertion of special catheters which are connected, usually via an electromechanical transducer, to a processor adapted to digitize and display the blood pressure reading taken by the transducer. This measurement is fairly traumatic and relatively problematical. Due to its invasive character, this measuring method is very seldom applied, and only to specific selected cases and at specially equipped intensive care centers. It should be emphasized, however, that this direct method does provide pressure readings which more accurately portray the real situation.
The non-invasive indirect method, also referred to as "palpation", is more widely adopted and is applied using a manual sphygmomanometer. In this method, the physician or operator directly feels (using a stethoscope to intensify his own auditory perception) the pulsating brachial artery at the elbow pit after applying an inflatable armband around a portion of the patient's arm. In some cases, the measurement can be made at the wrist on the radial artery by suitably repositioning the inflatable armband.
The blood pressure measuring begins with air being pumped into the inflatable armband to a pressure exceeding the systolic pressure by a safe margin. In this condition, the pulsation is subdued. The situation corresponds to having the vessel choked by the compressive force of the inflated armband. The measuring operation is then continued by gradually deflating the armband. This gradual deflation enables the operator to recognize certain characteristic sounds, referred to as Korotkoff's sounds, produced by the intermittent flow of blood through the now released vessel. Korotkoff's sounds gradually attain a maximum, to then fade out as the pressure from the inflated armband approaches the diastolic pressure value. Whereupon the blood through the vessel will cease to flow at an intermittent rate. The pressure readings are displayed directly on a special type of pressure gauge, usually carried on the inflatable armband itself.
The detection of incipient Korotkoff's sounds provides a first significant value, namely the value of the systolic pressure. The detection of their peak value provides a second significant value called the diastolic pressure value. The measurement of the first value, which is the maximum pressure value, is made with the manual sphygmomanometer and is quite accurate and reliable. However, the sound detection performed with manual sphygmomanometers is heavily dependent on the skill and experience of the operator in charge of making the measurement, and deeply affected by ambient noise. Electronic apparatus are also available commercially for measuring these particular physical signals. These apparatus are generally known as "electronic sphygmomanometers" and are effective in automating some of the steps of the non-invasive measuring method performed by manual sphygmomanometers.
An electronic sphygmomanometer basically comprises an inflatable armband which can be inflated and deflated around a patient's arm, an inflation pump, and an apparatus for measuring the physical signals of medical interest, such as those mentioned above. This apparatus incorporates a display screen for displaying the readings taken of the physical signals to be monitored. With an electronic sphygmomanometer, the inflatable armband is usually deflated automatically, and inflated manually by the operator using an ordinary pump. It is only with some of these apparatus that the armband deflation is also controlled automatically by the apparatus.
An example of an electronic sphygmomanometer is illustrated in U.S. Pat. No. 5,156,158, issued to Shirasaki on Oct. 20, 1992. This patent discloses a device as described above, which employs in particular, a fuzzy logic control unit capable of processing cardiovascular information by comparison to stored standard information using a plurality of membership functions. Shirasaki's electronic sphygmomanometer can speed up the step of becoming aware of such cardio- vascular information from the pressure of the sphygmomanometer inflatable armband.
Another prior non-invasive measuring method is the ultrasonic method, wherein an ultrasound generating/detecting apparatus is used. This ultrasonic apparatus can evaluate local movements of artery walls being measured. The blood flow is in fact related to variations in frequency of the ultrasonic reflections from the artery walls by the well-known Doppler effect. The ultrasonic measuring method has, however, a disadvantage in that it provides readings which overestimate the pressure, especially the systolic pressure value. Accordingly, this method is seldom used.
Automated medical equipment of this type is generally regarded as fundamentally unreliable. It is for this reason that the use of automatic measuring instruments is circumscribed in the medical field. Such apparatus are rejected by physician and hospitals on the grounds of their alleged unreliability. Physicians prefer to use well known manual sphygmomanometers, especially the mercury types, to feel more sure of their results.
In addition, manual sphygmomanometers allow details of the measurement to be assessed which represent important aspects to the evaluation of the reading correctness. However, a series of precautions must be taken with a fully manual apparatus, such as a manual sphygmomanometer, before a correct measurement can be made. This measurement, moreover, requires deep concentration and great care on the part of the operator who is performing the measuring operations manually.
In the indirect measuring method using a manual sphygmomanometer, the procedure outlined herein below is to be followed exactly. With the patient in a horizontal position, the inflatable armband inflating step is commenced with the armband being suitably placed around a portion of the patient's limb to be used in the measurement. An ordinary pump is associated with the armband. The inflating step should not be carried out at an excessively fast rate, and should not produce too high a compressive force so as not to inflict painful sensations on the patient which would result in disturbed pressure readings. In particular, it is found that an optimum rate for this step would be a rise of about 6 mmHg/s in a mercury column suitably linked to the inflatable armband.
Upon choking off the vessel involved in the measurement, the inflatable armband deflating step is commenced by releasing a manual exhaust screw. The patient is still in the horizontal position. Just like the inflating step, the deflating step should not be too rapid so as not to incur an underestimate of the systolic pressure value by stifling the first tones heard upon releasing the vase. Nor should the deflating step be too slow so as not to alter the pressure readings by inducing venous congestion. In this case, a secondary rise would occur in the diastolic pressure and the systolic pressure value would be under-estimated.